A fuel-filler tube for an automobile has been made of a welded steel pipe, and a fuel-supply opening is formed at its end. The fuel-supply opening is formed by pressing with a punch or bulging process to expand an end of a welded steel pipe, but the steel pipe is often cracked at the formed part. In this consequence, there is a strong demand for provision of a welded steel pipe with good formability.
The fuel-filler tube is installed in an automobile, in the state that it is coupled with a fuel tank. If the fuel-filler tube has poor airtightness, evaporated gasoline diffuses to the open air. Diffusion of gasoline shall be avoided for maintenance of clean atmosphere, but can not be inhibited by use of a conventional fuel-filler tube made of synthetic resin. Another type of a fuel-filler tube, which is made of a plain steel pipe expanded at its end, coated with a chromium layer and further coated with a paint film, is not always protected from corrosion, when it is exposed to a corrosive atmosphere such as a salty area. Corrosion also occurs inside the fuel-filler tube, which is exposed to a corrosive atmosphere containing an organic acid such as denatured gasoline or alcoholic fuel, and causes occurrence of pitting and opened holes in the end. Consequently, the fuel-filler tube drastically looses airtightness.
In order to overcome these disadvantages, applicability of stainless steel, i.e. a representative corrosion-resistant material to a fuel-filler tube, has been researched and examined for maintenance of airtightness over a long term. Stainless steel is well resistant to corrosion without necessity of plating or painting, but hard and easily work-hardened compared with plain steel. Due to the materialistic characteristics, a stainless steel pipe is difficult to form to a predetermined shape without cracks at its expanded part.
By the way, fuel-filler tubes, which are made of an expanded steel pipe with small diameter, are sometime used in response to lightening automobiles. However, a fuel-supply opening is unchanged in size with about 50 mm or so in inner diameter, regardless the size of a steel sheet. Since a steel pipe is necessarily formed at its end with great expansion ratio in this case, excellent formability of steel material is strongly demanded.
However, there is no proposal for provision of a stainless steel pipe, which exhibits good expansibility enough to be formed to a product shape as well as corrosion-resistance necessary for the purpose.